O LevelChemistryCambridgeTopic Questions11. Organic Chemistry11.6 Alcohols

Alcohols (Cambridge O Level Chemistry)

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1a5 marks

Islay is an island off the west coast of Scotland. The main industry on the island is making ethanol from barley.

Barley contains the complex carbohydrate, starch. Enzymes catalyse the hydrolysis of starch to a solution of glucose.

Yeast cells are added to the aqueous glucose. Fermentation produces a solution containing up to 10% of ethanol.
 
i)
Complete the word equation for the fermentation of glucose.
 
glucose → .......................... + ..........................
 
[1]
 
ii)
Explain why is it necessary to add yeast and suggest why the amount of yeast in the mixture increases.
 
[2]
 
iii)
Fermentation is carried out at 35°C. For many reactions a higher temperature would give a faster reaction. Why is a higher temperature not used in this process?
 
[2]

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1b3 marks
The organic waste, the residue of the barley and yeast, is disposed of through a pipeline into the sea. In the future this waste will be converted into biogas by the anaerobic respiration of bacteria. Biogas, which is mainly methane, will supply most of the island’s energy.
 
i)
Anaerobic means in the absence of oxygen. Suggest an explanation why oxygen must be absent.
 
[1]
 
ii)
The obvious advantage of converting the waste into methane is economic.
 
Suggest two other advantages.
 
[2]

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1a4 marks

Plants can make complex molecules from simple starting materials, such as water, carbon dioxide and nitrates. Substances produced by plants include sugars, more complex carbohydrates, esters, proteins, vegetable oils and fats.

i)
Describe how you could decide from its molecular formula whether a compound is a carbohydrate.
 
[2]
 
ii)
Plants can change the sugar, glucose, into starch which is a more complex carbohydrate. What type of reaction is this?
 
[2]

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1b5 marks

The fermentation of glucose can be carried out in the apparatus shown below. After a few days the reaction stops. A 12% aqueous solution of ethanol has been produced.

 
fermentation-of-glucose
 
i)
The enzyme, zymase, catalyses the anaerobic respiration of the yeast.
 
Explain the term respiration.
 
[2]
 
ii)
Complete the equation.
 
C6H12O6 → ......................... + ...................................
glucose        carbon dioxide
 
[2]
 
iii)
Why must air be kept out of the flask?
 
[1]

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1c2 marks

The ester methyl butanoate is found in apples. It can be made from butanoic acid and methanol. Their structural formulae are given below.

 
butanoic-acid methanol
butanoic acid  methanol
 

Use the information given above to deduce the structural formula of methyl butanoate showing all the bonds.

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1d3 marks

Proteins are natural macromolecules. Draw the structural formula of a typical protein. Include three monomer units. You may represent amino acids by formulae of the type drawn below.

 
amino-acid-representations
 

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2a5 marks

Butan-1-ol is used as a solvent for paints and varnishes, to make esters and as a fuel.
Butan-1-ol can be manufactured from but-1-ene, which is made from petroleum.

Biobutanol is a fuel of the future. It can be made by the fermentation of almost any form of biomass - grain, straw, leaves etc.

But-1-ene can be obtained from alkanes such as decane, C10H22, by cracking.

i)
Give the reaction conditions.

[2]

ii)
Complete an equation for the cracking of decane, C10H22, to give but-1-ene.

C10H22 →

[2]

iii)
Name the reagent that reacts with but-1-ene to form butan-1-ol.

[1]

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2b4 marks
i)
Balance the equation for the complete combustion of butan-1-ol.

...... C4H9OH + ...... O2 → ...... CO2 + ...... H2O

[2]

ii)
Write a word equation for the preparation of the ester butyl methanoate.

[2]

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2c3 marks

The fermentation of biomass by bacteria produces a mixture of products which include biobutanol, propanol, hydrogen and propanoic acid.

i)
Draw the structural formula of propanol and of propanoic acid. Show all the bonds.

[2]

ii)
Why is it important to develop these fuels, such as biobutanol, as alternatives to
petroleum?

[1]

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2d1 mark

How could you show that butanol made from petroleum and biobutanol are the same chemical?

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1a4 marks

Propane reacts with chlorine to form a mixture of chloropropanes. This is a photochemical reaction.

i)
What is meant by the phrase photochemical reaction?

[1]

ii)
The products of this reaction include two isomers, one of which has the following structural formula.

1-chloropropane

   Draw the structural formula of the other isomer.

[1]

iii)
Explain why these two different compounds are isomers.

[2]

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1b
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Bond breaking is an endothermic change and bond forming is an exothermic change. 

Bond energy is the amount of energy in kJ / mol needed to break one mole of the specified bond.

propane--cl2

Use the following bond energies to determine whether this reaction is exothermic or endothermic. You must show your reasoning.

Bond Bond energies in kJ / mol
C-Cl 338
C-H 412
Cl-Cl 242
H-Cl 431
C-C 348

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1c5 marks
i)
Chloropropane can be hydrolysed to propanol, CH3CH2CH2OH, by sodium hydroxide.

Write the equation for this reaction.

[2]

ii)
Propanol can be dehydrated. It loses a water molecule to form a hydrocarbon.

Give the name and structural formula of this hydrocarbon.

[2]

iii)
Propanol is oxidised to a carboxylic acid by acidified potassium manganate(VII).

Deduce the name of this acid.

[1]

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1d
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Propanol reacts with methanoic acid to form the ester propyl methanoate.


CH3CH2CH2OH + HCOOH → HCOOCH2CH2CH3 + H2O

4.0 g of methanoic acid was reacted with 6.0 g of propanol.

i)
Calculate the Mr of methanoic acid.

[1]

ii)
Calculate the Mr of propanol.

[1]

iii)
Determine which one is the limiting reagent. Show your reasoning.

[2]

iv)
Calculate the maximum yield in grams of propyl methanoate, Mr = 88.

[1]

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2a
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The alcohols form a homologous series.

i)
Give three characteristics which all members of a homologous series share.
[3]
ii)
Give the name of the third member of this series.
[1]
iii)
Deduce the molecular formula of the alcohol whose Mr = 158. Show your working.
[2]

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2b2 marks

Explain why the following two alcohols are isomers.

alcohol-isomers

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2c5 marks

This question is based on typical reactions of butan-1-ol.

i)
When butan-1-ol, CH3–CH2–CH2–CH2–OH, is passed over the catalyst silicon(IV) oxide, water is lost.

Deduce the name and the structural formula of the organic product in this reaction.

[2]

ii)
Suggest the name of the ester formed from butanol and ethanoic acid.

[1]

iii)
Butan-1-ol is oxidised by acidified potassium manganate(VII).

Deduce the name and the structural formula of the organic product in this reaction.

[2]

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3a2 marks

Alcohols can be made by fermentation or from petroleum.

Ethanol can be made by the fermentation of glucose.

C6H12O6 (aq) rightwards arrow with yeast on top2C2H5OH (aq) + 2CO2 (g)       exothermic reaction

Yeast are living single-cell fungi which ferment glucose by anaerobic respiration. This reaction is catalysed by enzymes from the yeast.

i)
What are enzymes?

[1]

ii)
Suggest a method of measuring the rate of this reaction.

[1]

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3b4 marks

The following observations were noted.

  • When a small amount of yeast was added to the aqueous glucose the reaction started and the solution went slightly cloudy.
  • The reaction rate increased and the solution became cloudier and warmer.
  • After a while, the reaction rate decreased and eventually stopped, leaving a 14% solution of ethanol in water.
i)
Why did the reaction rate increase?

[1]

ii)
Suggest an explanation for the increase in cloudiness of the solution.

[1]

iii)
Give two reasons why the fermentation stopped.

[2]

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3c2 marks

One use of ethanol is in alcoholic drinks.

Give two other uses of ethanol.

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3d5 marks

Alcohols can be made from petroleum by the following sequence of reactions.

alkanes from petroleum → alkene → alcohol

Describe the manufacture of ethanol from hexane, C6H14. Include in your description an equation and type of reaction for each step.

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4a6 marks

The structural formula of a butanol is given below.


CH3—CH2—CH2—CH2—OH

Butanol can be made from petroleum and also by fermentation.

i)
Describe the chemistry of making butanol from petroleum by the following route.

petroleum → butene → butanol

[3]

ii)
Explain, in general terms, what is meant by fermentation.

[3]

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4b2 marks

Butanol can be oxidised to a carboxylic acid by heating with acidified potassium manganate(VII). Give the name and structural formula of the carboxylic acid.

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4c4 marks

Butanol reacts with ethanoic acid to form a liquid, X, which has the sweet smell of bananas. Its empirical formula is C3H6O and its Mr is 116.

i)
What type of compound is liquid X?

[1]

ii)
Give the molecular formula of liquid X.

[1]

iii)
Draw the structural formula of X. Show all the individual bonds.

[2]

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5a3 marks

The alcohols form an homologous series.

Give three characteristics of a homologous series.

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5b3 marks

The following two alcohols are members of the series and they are isomers.

 

CH3—CH2—CH2—CH2—OH and (CH3)2CH—CH2OH

i)
Explain why they are isomers.

[2]

ii)
Give the structural formula of another alcohol which is also an isomer of these alcohols.

[1]

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5c3 marks

Copper(II) oxide can oxidise butan-1-ol to liquid X whose pH is 4.

 

cie-igcse-sq-11-2-practical

i)
Name another reagent which can oxidise butan-1-ol.

[1]

ii)
What type of compound is liquid X and what is its formula?

[2]

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5d6 marks

The alcohol ethanol can be made by fermentation. Yeast is added to aqueous glucose.

C6H12O6 (aq) → 2C2H5OH (aq) + 2CO2 (g)

Carbon dioxide is given off and the mixture becomes warm as the reaction is exothermic.
The graph shows how the rate of reaction varies over several days.

cie-igcse-sq-11-2-graph

i)
Suggest a method of measuring the rate of this reaction.

[2]

ii)
Why does the rate increase initially?

[1]

iii)
Suggest two reasons why the rate eventually decreases.

[2]

iv)
Why is fermentation carried out in the absence of air?

[1]

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6a3 marks

The alcohols form a homologous series. Two characteristics of a homologous series are that the physical properties of the members vary in a predictable way and they have similar chemical properties.

Complete the table.

name formula mass of one mole / g boiling point / oC
methanol CH3 –OH 32 64
ethanol CH3 –CH2 –OH 46 78
propan-1-ol CH3 –CH2 –CH2 –OH 60 98
butan-1-ol CH3 –CH2 –CH2 –CH2 –OH 74 118
pentan-1-ol     138
hexan-1-ol CH3 –CH2 –CH2 –CH2 –CH2 –CH2 –OH 102  

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6b2 marks

Give two other characteristics of a homologous series.

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6c3 marks

Draw a diagram showing the arrangement of the valency electrons in one molecule of the covalent compound methanol.

Use x to represent an electron from a carbon atom.
Use ● to represent an electron from an oxygen atom.
Use o to represent an electron from a hydrogen atom.

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6d3 marks

Alcohols can be oxidised to carboxylic acids by heating with acidic potassium manganate(VII).

i)
Draw the structural formula of the carboxylic acid formed by the oxidation of propan-1-ol. Show all the bonds.

[1]

ii)
Describe how ethanol could be oxidised to ethanoic acid by fermentation.

[2]

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6e2 marks

Propan-1-ol and ethanoic acid react together to form an ester. Give its name and structural formula.

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7a
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The alcohols form a homologous series. The first four members are methanol, ethanol, propan-1-ol and butan-1-ol.

One characteristic of a homologous series is that the physical properties vary in a predictable way. The table below gives the heats of combustion of the first three alcohols.

Alcohol Formula Heat of combustion in kJ / mol
methanol CH3OH -730
ethanol CH3-CH2-OH -1370
propan-1-ol CH3-CH2-CH2-OH -2020
butan-1-ol CH3-CH2-CH2-CH2-OH  

i)
The minus sign indicates that there is less chemical energy in the products than in the reactants. What form of energy is given out by the reaction?

[1]

ii)
Is the reaction exothermic or endothermic?

[1]

iii)
Complete the equation for the complete combustion of ethanol.

[2]

iv)
Determine the heat of combustion of butan-1-ol by plotting the heats of combustion of the first three alcohols against the number of carbon atoms per molecule.

graph-heat-of-combustion

[3]

v)
Describe two other characteristics of homologous series.

[2]

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7b2 marks

Give the name and structural formula of an isomer of propan-1-ol.

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7c6 marks

Methanol is made from carbon monoxide.

CO (g) + 2H2(g)rightwards harpoon over leftwards harpoon CH3OH (g) the forward reaction is exothermic

i)
Describe how hydrogen is obtained from alkanes.

[2]

ii)
Suggest a method of making carbon monoxide from methane.

[2]

iii)
Which condition, high or low pressure, would give the maximum yield of methanol?
Give a reason for your choice.

[2]

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7d3 marks

For each of the following predict the name of the organic product.

i)
reaction between methanol and ethanoic acid

[1]

ii)
oxidation of propan-1-ol by potassium dichromate(VI)

[1]

iii)
removal of H2O from ethanol (dehydration)

[1]

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